The invention relates to a system and method for controlling dual camshafts in a variable cam timing engine.
Known engines have utilized variable cam timing (VCT) mechanisms to control the opening and closing of intake valves and exhaust valves communicating with engine cylinders. In particular, each VCT mechanism is utilized to adjust a position of a camshaft (which actuates either an intake valve or exhaust valve or both) with respect to a crankshaft position. By varying the position of the camshaft (i.e., camshaft angle) with respect to the position of the crankshaft, engine fuel economy can be increased and engine emissions can be decreased.
In known engines having VCT mechanisms, it is desired to shift the position of camshafts in the VCT mechanisms synchronously (i.e., at the same speed) to a desired phase angle with respect to the crankshaft. However, the inventors herein have recognized that first and second camshafts associated with first and second VCT mechanisms, respectively, in an engine, may not move to the desired phase angle at the same speed. For example, the first VCT mechanism may be actuated at a lower pressure that a second VCT mechanism due to a clogged oil line communicating with the first VCT, resulting in slower movement of the first camshaft. Still further, the first VCT mechanism may xe2x80x9cstickxe2x80x9d at cold temperatures resulting in slower movement of the first camshaft as compared to the second camshaft of the second VCT mechanism. During non-synchronous movement of the first and second camshafts, the air charge delivered to first and second cylinder banks, respectively, are different. The difference in air charge can result in a differing torques being produced by the first and second cylinder banks resulting in undesirable engine shaking and increased engine no se. Further, the difference in air charge may result in non-optimal spark timing in one of the cylinder banks resulting in increased engine knock in the cylinder bank. Still further, the difference in air charge may result in a rich air-fuel mixture being delivered to one of the cylinder banks resulting in decreased fuel economy.
The inventors herein have recognized that there is a need for a system and method for synchronizing the movement of first and second camshafts of an engine to reduce and/or eliminate the above-mentioned deficiencies.
The foregoing problems and disadvantages are overcome by a system and method for controlling first and second camshafts in a variable cam timing engine. The first and second camshafts control air flow communicating with first and second cylinders, respectively, of the engine. The engine further includes a crankshaft driven by first and second pistons within the first and second cylinders, respectively. The inventive method includes determining when the first camshaft is moving toward a first scheduled phase angle with respect to the crankshaft at a faster rate than the second camshaft is moving toward the first scheduled phase angle. The method further includes slowing down the first camshaft so that the rate of movement of the first camshaft approaches a rate of movement of the second camshaft toward the first scheduled phase angle.
A system for controlling first and second phase shiftable camshafts in a variable cam timing engine is also provided. The system includes a first sensor generating a first signal indicative of a position of the first camshaft. The system further includes a second sensor generating a second signal indicative of a position of the second camshaft. The system further includes a third sensor generating a third signal indicative of a position of the crankshaft. The system further includes a controller operably connected to the first, second, and third sensors. The controller is configured to determine when the first camshaft is moving toward a first scheduled phase angle with respect to the crankshaft at a faster rate than the second camshaft is moving toward the first scheduled phase angle based on the first, second, and third signals. Finally, the controller is configured to slow down the first camshaft so that the rate of movement of the first camshaft approaches the rate of movement of the second camshaft toward the first scheduled phase angle.
The inventive system and method for controlling the first and second camshafts solves the problem of engine torque fluctuations during movement of the camshafts. In particular, the inventive system and method slows down the movement of the faster camshaft so that the first and second camshafts move at approximately the same speed toward a desired phase angle. The synchronous movement results in an equal air charge being provided to first and second cylinder banks during the dual camshaft movement which reduces the engine torque fluctuations.